A new study estimates the total amount of carbon currently accumulating in the topsoil of US forests undergoing two types of reforestation: actively replanting trees after distances like wildfires, or allowing forests to retake marginal croplands.

Forests across the United States—and especially forest soils—store massive amounts of carbon, offsetting about 10 percent of the country’s annual greenhouse gas emissions and helping to mitigate climate change.

But for more than 20 years, experts have warned that the strength of this carbon “sink” is declining and will level off around mid-century.

The new research also looks at the potential to expand carbon sequestration in reforesting areas.

“Where reforestation is happening—either through planting of trees or through encroachment—these lands are actively adding carbon to a large pool that will continue to grow for many decades,” says lead author Luke Nave, ecologist and biogeochemist at the University of Michigan.

“The topsoils of reforesting lands provide a significant long-range solution to the problem of the declining carbon-sink strength of US forests, and they help to mitigate climate change. Even modest increases in the amount of land being reforested would have a multiplicative impact on nationwide carbon sequestration.”

Tons and tons

The researchers found that reforesting topsoils across the country are currently adding 13 million to 21 million metric tons (13-21 teragrams) of carbon each year, an amount equivalent to about 10 percent of the total US forest-sector carbon sink and offsetting about 1 percent of all US greenhouse gas emissions.

Over the next century, reforesting US topsoils will sequester a cumulative 1.3 to 2.1 billion metric tons (1.3-2.1 petagrams) of carbon, accounting for nearly half of the soil-carbon gains occurring on US forestland, says Nave, an assistant research scientist at the University of Michigan Biological Station and in the department of ecology and evolutionary biology.

And the amount of stored carbon could increase dramatically if the nation’s reforesting acreage, currently at nearly 200,000 square miles, grows.

So much potential

As part of the study, the researchers looked at US forestlands that have experienced major disturbances, such as intense wildfires or severe insect outbreaks, using National Forest Inventory data from the last several decades. They found that only about 7 percent of the forestlands available for replanting have been replanted.

When they looked at marginal croplands undergoing reforestation, they found that carbon-storage gains to date are only about 10 percent of their potential. That finding highlights “the substantial C-sink capacity of this land-use transition if these lands are allowed to continue returning towards a natural forest condition,” the authors write.

For their study, the researchers combined satellite imagery with some 15,000 on-the-ground measurements of topsoil carbon from two national-level databases. One of the databases, from the International Soil Carbon Network, includes soil-carbon measurements at the university’s Biological Station near Pellston.

Coauthors of the paper are from the US Department of Agriculture-Forest Service; Cornell University; and the Argonne National Laboratory.

The Department of Agriculture-Forest Service and the National Science Foundation supported the work, which appears in PNAS.